1). Field of the Invention
The present invention is concerned with control of a motor vehicle powertrain having a continuously variable transmission of torque-controlled type.
2). Discussion of Related Art
In any continuously variable transmission there is a unit, referred to herein as a “variator”, which provides a continuously variable drive ratio. The variator couples to other parts of the transmission—typically gearing leading on one side of the variator to the engine and on the other side to driven components such as the driven wheels of a motor vehicle—through rotary input and output members. The ratio of rotational speeds of the variator's input and output members is the “variator drive ratio”. The overall ratio of speeds of input and output members of the transmission itself—coupled to the engine and the driven wheels respectively—is the “transmission ratio”.
The concept of “torque control” is known in this art but will now be explained. It is useful to distinguish torque control from the alternative of “ratio control”.
A ratio-controlled variator receives a control signal representing a required variator drive ratio. The variator responds by adjusting its drive ratio to the required value. The adjustment typically involves detecting the position of a ratio-determining element of the variator (e.g. the separation of the sheaves in a belt-and-sheave variator, or the position of the rollers in a toroidal-race type variator) and adjusting the actual position of this element to a desired position (determined by the control signal) using a feedback loop. Thus in a ratio controlled variator, ratio is a function of the control signal.
This is not the case in a torque-controlled variator. Instead a torque-controlled variator is constructed and arranged such as to exert upon its input and output members a reaction torque which corresponds directly to the control signal. It is torque which is the control variable rather than drive ratio. Changes in speed of the variator input and output, and hence changes in variator drive ratio, result from the application of these torques, added to the externally applied torques (e.g. from engine and wheels), to the inertias coupled to the variator input and output. The variator drive ratio is permitted to change correspondingly. “Reaction torque”, as used herein, is the sum of the torques exerted by the variator upon its input and output members.
The challenges involved in controlling a torque-controlled transmission are very different from those involved in controlling a ratio-controlled transmission. In the latter, since the variator maintains a chosen drive ratio, torque at the driven wheels is related directly to engine torque. Engine speed control is a relatively straightforward matter since, by maintaining a set drive ratio, the transmission provides a direct relationship between engine speed and vehicle speed. In a torque controlled transmission, in which drive ratio is not the control variable and is permitted to vary, the engine and wheels can be thought of as being effectively de-coupled from one another. Wheel torque is controlled by the variator rather than by engine torque. Engine speed is not constrained to follow vehicle speed. Instead the control signal applied to the variator determines a loading torque applied by the variator to the engine. Combustion within the engine creates an engine torque. The sum of loading torque and engine torque acts upon the inertia referred to the engine (contributed by masses in both engine and the transmission) and so determines engine acceleration. While the loading torque and the engine torque are equal and opposite, engine speed is constant. Changes in engine speed result from an inequality between these torques. Dynamic matching of engine torque to loading torque is thus fundamental to management of the drive line as a whole and of engine speed in particular. Failure to manage the balance would allow unwanted changes in engine speed.
Effective utilization of torque-controlled transmissions depends on electronics to regulate the engine and transmission in unison. Early papers on the electronic control of such a powertrain are by Stubbs—“The Development of a Perbury Traction Transmission for Motor Car Applications”, ASME (The American Society of Mechanical Engineers) paper no. 80-GT-22, March 1980 and also by Ironside and Stubbs “Microcomputer Control of an Automotive Perbury Transmission”, IMechE paper no. C200/81, 1981. Both papers describe a project concerned with electronic control of a transmission based on a toroidal-race rolling-traction type variator operating in torque-controlled mode. The driver's demand was interpreted as a requirement for wheel torque, which was then converted into a requirement for engine power by multiplication by the rotational speed of the vehicle wheels. From this power, target values for the engine torque and engine speed were selected. The engine was to produce the target torque, and the variator reaction torque was adjusted to bring the engine speed to the target value, using a closed loop based on engine speed. In this scheme the target engine torque and speed are the values at which the engine would desirably be run in a “steady state” where engine speed was constant. In this steady state the target engine speed and torque, acting through the transmission, would provide the wheel torque demanded by the driver. Stubbs' simple approach to management of engine speed proves inadequate in practice. The transmission ratio is prone to oscillation and the vehicle lacks “driveability”. The adjustment of variator reaction torque used to control engine speed means that, when engine acceleration takes place, the actual wheel torque differs from the driver's demand.
A more sophisticated approach involves setting the engine torque not to its target value but instead to a value modified to take account of the engine torque required to accelerate powertrain inertias contributed by rotary components of the engine and transmission. In this way the engine contributes to control of the aforementioned dynamic balance. Calculation of the necessary engine torque has hitherto been based upon a required acceleration of the engine. It has now been recognised, however, that in setting the engine torque account should additionally be taken of the acceleration of the motor vehicle itself.